1. Field of the Invention
The present invention relates to a field effect transistor.
2. Description of the Related Art
FIGS. 1A to 1D are cross sectional views of a field effect transistor in a conventional method of manufacturing the field effect transistor.
Referring to FIG. 1A, an intrinsic type GaAs buffer layer 2 with high purity is formed on a GaAs semi-insulating substrate 1. Subsequently, an intrinsic type In.sub.0.15 Ga.sub.0.85 As channel layer 3 with high purity, a Si-doped n-type Al.sub.0.2 Ga.sub.0.8 As electron supply layer 4, and a Si-doped n-type GaAs cap layer 5 are laminated on the GaAs buffer layer 2 in this order.
Subsequently, as shown in FIG. 1B, the Si-doped n-type GaAs cap layer 5 is etched using a photoresist layer 6 so as to expose a part of the Si-doped n-type Al.sub.0.2 Ga.sub.0.8 As electron supply layer 4.
Subsequently, as shown in FIG. 1C, a Ti layer, a Pt layer and an Au layer (Ti/Pt/Au) are laminated on the exposed part of the Si-doped n-type Al.sub.0.2 Ga..sub.0.8 As electron supply layer 4 in this order to form Schottky gate electrode 7.
Next, as shown in FIG. 1D, an AuGe layer, a Ni layer and an Au layer (AuGe/Ni/Au) are laminated in this order on the Si-doped n-type GaAs cap layer 5 on both sides of the Schottky gate electrode 7 to form ohmic source and drain electrodes 8. Subsequently, heat treatment (alloy) is performed to form ohmic alloy layers 9.
This type of field effect transistor formed on the GaAs substrate is called a HJFET (Hetero Junction Field Effect Transistor). In this transistor, electrons generated from the Si-doped n-type Al,.sub.0.2 Ga.sub.0.8 As electron supply layer 4 are accumulated as a two-dimensional electron gas in the high purity In.sub.0.15 Ga.sub.0.85 As channel layer 3 which is spatially separated from donor atoms. Therefore, the electrons has very high mobility.
FIG. 2 shows a graph showing an electrostatic potential under the Schottky gate electrode of the field effect transistor. In the conventional GaAs HJFET described above, the gate electrode is formed directly on the Si-doped n-type Al.sub.0.2 Ga.sub.0.8 As layer 4 which is the electron supply layer. For this reason, the inclination of the electrostatic potential under the gate becomes steeper. In the field effect transistor having such a structure, when the film thickness of the Si-doped n-type Al,.sub.0.2 Ga.sub.0.8 As electron supply layer 4 changes in the wafer surface due to epitaxial growth and crystal etching, the threshold voltage of the field effect transistor changes largely even if the change quantity of the film thickness is small, because the inclination of the electrostatic potential near the surface is steep. As a result, in the conventional field effect transistor, there is a problem in that the uniformity and reproducibility are low.